Technical Abstract:
Global warming has been predicted as a consequence of the Earth’s rising atmospheric CO2 concentration. Both the higher concentrations of CO2 and higher temperatures are likely to affect the productivity of agricultural crops in the future. To study these phenomena under natural field conditions without the artifacts imposed by chamber walls, scientists have developed free-air CO2 enrichment (FACE) technology for researching the effects of elevated CO2. Increasing temperatures of open-field plots, however, is even more problematic, and few attempts have been made. Progress in development of arrays of infrared heaters for T-FACE (temperature free-air controlled enhancement) will be presented. A governing equation for predicting performance from weather and crop parameters was derived and validated from measured performance data. Using the equation with hourly weather data allows one to estimate how much heater power will be required to achieve a desired warming regime for an acceptable percentage of a growing season. As would be expected, the windiness of a site is a most important factor in determining costs of running a T-FACE experiment. The most powerful and efficient infrared heaters commercially available appear to be those with a ceramic heating element, but the thermal energy distribution from individual heaters does not form a uniform pattern. However, by tilting these heaters at 45 degrees and combining several into an array, improved uniformity of the thermal radiation and of canopy temperature across 3.0-m-diameter plots was achieved. In particular, measurements with hexagonal and octagonal arrays had excellent uniformity. Moreover, the heaters, which were suspended from cables around the periphery of the plot at about 1.2 m above the vegetation, only shaded the plots about 1-2%.